1. Field of the Invention
The present invention generally relates to imbedding temperature-sensing circuits into integrated circuits. More particularly, this invention relates to building more accurate, lower power, and smaller integrated circuit area temperature sensors.
2. Description of the Prior Art
Temperature sensors are used to control various integrated circuit functions to control various integrated circuit functions. These dynamic functions include random access memories (DRAM) refresh frequency and delay chain delay time, both of which vary with temperature. On-chip temperature sensors are used to regulate or vary the amount of DRAM refresh applied as a function of temperature. Similarly, on-chip temperature sensors are used to regulate or stabilize circuit delay time variations, which occur. This stabilization of circuit delay time is critical for circuits, which depend on the accuracy of circuit delay for correct circuit applications, such as circuit delay chain circuits. In addition, on-chip temperature sensors are desired in order to implement digital thermometer applications.
Since temperature sensors are sharing parts of integrated circuits with other integrated functions, it is important that these integrated temperature sensors occupy minimal chip area and consume minimal chip power. In addition, another important design parameter for integrated temperature sensors are the accuracy of the temperature measurement itself.
FIG. 1a shows a prior art diagram of voltage versus temperature. FIG. 1a graphs voltage versus temperature for 4 nodes pictured in the circuit of FIG. 1c. As shown in FIG. 1a, the intersection of the VR1 straight-line graph and the Vbe2 curve occurs at temperature, T1. VR1 is the voltage at the top node of resistor, R1 in FIG. 1c. Vbe2 is the voltage across transistor, Q2. Voltage, Vbe2, is the reference voltage, VREF=Vbe2, in FIG. 1c. VR2 is the voltage at the top node of resistor, R2, in FIG. 1c. 
FIG. 1c shows Vbe2 and VR1 as inputs to a comparator amplifier 110. FIG. 1c shows Vbe2 and VR2 as inputs to a comparator amplifier 120. FIG. 1c shows Vbe2 and VR3 as inputs to a comparator amplifier 130. VR3 is the voltage at the top node of resistor, R3, in FIG. 1c. 
In FIG. 1a, if VR1 is larger than Vbe2, then VT1 will be non-zero. A non-zero VT1 indicates that the circuit of FIG. 1c detected a temperature range above T1 as shown in FIG. 1b. 
In FIG. 1a, if VR2 is larger than Vbe2, then VT2 will be non-zero. A non-zero VT2 indicates that the circuit of FIG. 1c detected a temperature range above T2 as shown in FIG. 1b. 
In FIG. 1a, if VR3 is larger than Vbe2, then VT3 will be non-zero. A non-zero VT3 indicates that the circuit of FIG. 1c detected a temperature range above T3 as shown in FIG. 1b. 
U.S. Pat. No. 6,078,208 (Nolan et al.) describes a precision temperature sensor which produces a clock frequency which varies over wide variations of ambient temperature. The circuit has an oscillation generator, two independent current generators, a reference oscillator and a frequency counter. The outputs of the two independent current generators are combined to provide an approximately linear capacitor charging current which is directly proportional to changes in temperature. The capacitor charging current is used to drive the oscillation generator which outputs a clock frequency that is linearly dependent on temperature with determinable slope and intercept. The frequency counter compares the output of the oscillation generator with the reference oscillator to compute a digital value for temperature.
U.S. Pat. No. 6,019,508 (Lien) discloses an integrated temperature sensor circuit. This circuit comprises two different current sources multiplexed using switches which are controlled by clocks having opposite phases. A first voltage is developed on a capacitor during a first clock phase and a second voltage is developed on the capacitor during the second clock phase. A second capacitor is coupled between the input and output of an operational amplifier. The second capacitor is discharged during the first clock phase and is charged during the second clock phase. Since the second voltage is dependent on temperature, the voltage at the output of the operational amplifier is dependent on the temperature and the ratio of the two capacitors.
U.S. Pat. No. 5,835,553 (Suzuki) describes a temperature sensor circuit. This circuit includes a pulse source for generating a count pulse and a resistor having a resistance changing dependently upon a temperature change. The temperature detecting circuit is designed to convert the change of the resistance of the resistor responding to the temperature change, into a number which represents the number pulses counted. A counter counts the count signal and accumulates a count value for each temperature-measuring signal so as to hold the accumulated count value. The counter outputs the accumulated count value in response to a reset signal having a second frequency lower than the first frequency.